The sex-specific factor SOA controls dosage compensation in Anopheles mosquitos.

The Anopheles mosquito is one of thousands of species in which sex differences play a central role in their biology, as only females need a blood meal in order to produce eggs. Sex differentiation is regulated by sex chromosomes, but their presence creates a dosage imbalance between males (XY) and females (XX). Dosage compensation (DC) can re-equilibrate the expression of sex-chromosomal genes, but because DC mechanisms have only been fully characterized in a few model organisms, key questions about its evolutionary diversity and functional necessity remain unresolved 1 . Here we report the discovery of a previously uncharacterized gene (SOA, for sex chromosome activation) as a master regulator of DC in the malaria mosquito Anopheles gambiae. Sex-specific alternative splicing prevents functional SOA protein expression in females. The male isoform encodes a DNA-binding protein that binds the promoters of active X chromosomal genes. Expressing male SOA is sufficient to induce DC in female cells. Male mosquitoes lacking SOA or female mosquitos ectopically expressing the male isoform exhibit X chromosome misregulation, which is compatible with viability but causes developmental delay. Thus, our molecular analysis of the first DC master regulator in a non-model organism elucidates the evolutionary steps leading to the establishment of a chromosome-specific fine-tuning mechanism.